A battery pack using a secondary battery is widely used as a power source of portable electronic equipment such as a cellular phone, a notebook personal computer (hereinafter called a notebook PC), and a PDA. The battery pack is appropriately provided with a function of protecting a secondary battery from over-charge, over-discharge, a short circuit, and the like, a function of calculating and displaying the amount of remaining capacity, a function of communicating with a charging device and a loading device, a function of detecting compatibility with a connected device or an imitation, and the like in accordance with a demand of the charging device or the loading device to which the battery pack is connected.
For example, as shown in FIG. 7, a battery pack 100 for a cellular phone is provided with a secondary battery 101, a battery protection IC 102 and a protection element 105 for protecting the secondary battery 101 from over-discharge, over-charge, over-current, and the like, a PTC element 103 for preventing a short circuit current, a thermistor 104 for detecting the temperature of the charged battery, and the like. The battery pack 100 connected to a charging device through connection terminals 106, 107, and 108 is charged while the thermistor 104 monitors the battery temperature. The battery pack 100 connected to a loading device (being a cellular phone) through the connection terminals 106 and 107 discharges battery energy to the loading device. A battery pack for a notebook PC adopts a smart battery system pursuant to a system management bus standard by which the battery pack and a PC body are connected to each other with a bidirectional two-wire bus or the like for allowing the notebook PC to manage a power source.
To prevent the failure or accident of the battery pack caused by connecting the battery pack to an incompatible charging device, to prevent a failure or an accident due to the use of an imitation battery pack, and the like, a battery pack is known which is provided with ID memory means on which identification information for identifying a connectable battery pack is stored. FIG. 8 shows the conventional structure of a battery pack and a connection system thereof in which a battery pack 200 is provided with an ID circuit section 207 and a connection device (being a loading device or charging device) 300 to which the battery pack 200 is connected is provided with an ID detection section 307 in order to prevent an imitation battery pack from being connected to a loading device and prevent the battery pack from being connected to a charging device except for a compatible exclusive charger.
In FIG. 8, the battery pack 200 is provided with a secondary battery 201, temperature detection means 202, voltage detection means 203, current detection means 204 for detecting each of a temperature, a voltage, a charging and discharging current of the secondary battery 201, a control circuit 206 which turns on and off a protection element 205 for control by detecting the state of over-charge, over-discharge, and over-current of the secondary battery 201 by detection information from each detection means, and the ID circuit section 207 on which identification information is stored. The positive electrode and negative electrode of the secondary battery 201 are connected to a positive charge and discharge terminal 208 and a negative charge and discharge terminal 209, respectively, and the ID circuit section 207 is connected to an ID terminal 210. The connection device 300, which refers to the charging device here, is provided with a charging power source 301, the ID detection section 307 for reading the identification information out of the ID circuit section 207, and a condition selection section 306 which judges the compatibility of the battery pack 200 from the identification information which is read out and controls the output of charging energy from the charging power source 301.
When not only the foregoing battery packs 100 and 200 but also any battery pack is connected to a charging device or a loading device, it is indispensable to connect an information transmission line for transmitting control information, ID information, and the like in addition to a direct-current power transmission line for the positive and negative electrodes. In other words, since a signal transmission line exists except for the two line connections essentially required for transmitting the direct-current power, three or more connection terminals are required for connecting the battery pack to the connection device. The more the numbers of the connection terminals increases, the more the connection terminals are susceptible to electromagnetic interference (EMI) and electrostatic destruction. Accordingly, there is the fear of influence such as degradation in function, a malfunction, the disappearance of stored information, and the like. Also, the device is difficult to miniaturize due to upsizing of the connection terminals and costs tend to increase due to the number of man-hours required for wiring.
Accordingly, it is preferable to reduce the number of connection terminals, and the structure of only two connection terminals for connecting the direct-current power transmission line which is essentially required is ideal. As a conventional technology for realizing this structure, in order to connect a battery pack to a charging device, a structure is known in which information is transmitted via an electromagnetic connection or an optical connection and there are only two positive and negative charge and discharge terminals for transmitting direct-current power as the connection terminals (see, for example, Patent Document 1).    [Patent document 1] Japanese Patent Laid-Open Publication No. 05-135804